Alternate energy systems such as solar photovoltaic (PV) and wind power face a challenge in that availability of energy supply is not synchronized with the demand for electricity. At present, there are a limited number of technologies available to store energy at the scale required for utility-level consumption (i.e. on the order of several megawatts). The current US electrical grid can tolerate approximately 10% alternate energy supply. The challenge in utility operations is to balance the available supply with demand. Given the present state of technology, short duration (minute time scale) load balancing is provided by fossil fuel fired power plants and, more particularly, by natural gas fired generators. To provide grid stability so that supply voltage to consumers is stable and to support a larger supply from alternate energy sources, a method must be developed for energy storage.
In the case of solar PV, the time duration for storage tends to be on the order of minutes. Solar PV output tends to peak during the day when the consumer demand tends to peak. Solar PV is subject to variable output due to clouds passing over the PV system.
In the case of wind power, there is an advantage in capital cost versus present solar PV technology. Wind power has a lower cost per unit power output than solar PV. Wind power suffers from two deficiencies. During an particular time period during the day, the wind generally experiences variance in strength on the order of seconds and minutes time scale. Furthermore, there is a diurnal variance in wind strength. In general winds tend to blow stronger at night than during the daytime. The availability of energy from the natural resource (i.e. the wind strength) is approximately twelve hours out of sync with ability of the grid to utilize this energy. Thus, there is a need for an energy storage means that transform the available energy from the wind at maximum efficiency, convert this energy into manageable form, and have a mechanism that can remove energy from this manageable form so that power output can respond to the second by second variance of consumer demand.
Due to normal climatic conditions, the wind tends to blow more strongly at night than during the day. Texas has an electric grid that is separate from the remainder of the US. In West Texas, the wind strength is suitable for driving wind turbines. Unfortunately, there is an excess of wind power generated at night. If the wind turbine operator generates power at night, they actually have to pay the utility. The utility is willing to buy power during the day; however, there is less wind power availability (due to lower wind speeds) during the daylight hours.
There is a need to decouple the potential to create power from the demand for the power, at least in time if not in location. This storage capacity need not extend over several days. It would be sufficient to store the generated power for approximately a 12 hour period. Also, in terms of efficiency, there is an advantage to optimizing the wind turbine controls to maximize the output of the turbine based on the current capacity of the wind, rather than limiting the turbines output based on the demand of the grid. This is why wind turbines are frequently seen not in operation. There may be wind available; however, there might be no instantaneous demand.
There are several different methods that have been piloted for energy storage:
Hydroelectric storage
Batteries
Compressed air
Flywheel
Hydrogen
Supercapacitors
These techniques are summarized at the following web sites: sandia.gov/ess/About/newsevents.html#arpa-e and er.doe.gov/bes/reports/abstracts.html#EES.
In general, the renewable energy or alternate energy produced needs to be stored at its point of creation. To locate the source and the storage mechanism at different physical locations separated by wires would require either shared use of the electrical grid or the capital investment of a dedicated grid. In the former case, the transmission of power over the shared grid further contributes to grid instability.
In brief, hydroelectric storage uses available power at low demand periods to pump water into an elevated reservoir. During periods of peak demand, the power is released form the reservoir and passed through a hydroelectric turbine. The energy storage capacity of such a system is determined by the difference in elevation and the mass of water moved. These systems have a low energy storage density and are significantly constrained by the geology and topography at the installed location.
Suppose one wanted to store 22.5 MW-hr of power that would correspond to a 2.5 MW wind turbine operating at 75% capacity for 12 hours. Storing this much energy at a rise in height of 10 m (32.8 ft) requires a total area of 236,000 m2 (23.6 hectares, or 59 acres). Storage depth would be 3.5 m (11.5 ft). A significant quantity of land would be required in the near vicinity of each wind turbine. Also, one wind turbine does not generate that much energy. A typical, small fossil fuel power plant would be at least 150 MW, the equivalent of more than sixty 2.5 MW wind turbines.
The Tennessee Valley Authority (TVA) built a hydroelectric storage system in the 1960s at Raccoon Mountain. (Attached is a brochure and the information from Wikipedia.) The TVA converted an entire mountain into a storage facility comprised of a reservoir, pumping means to lift water into the reservoir, and a spillway and turbine system to create power. The geology and topography of this installation is unique and would not typically be found in locations where there is wind availability and, hence, ample wind energy for a wind farm. Furthermore, it is unclear if an organization could get such a civil engineering structure permitted today due to the alternation of the natural environment.
For batteries, the total energy storage needed is beyond most known technologies. There are significant limitations to battery electrolyte chemistry. This area is a focus for on-going US Department of Energy efforts on energy storage. Currently, the only technology that has been piloted for near utility scale are sodium-sulfur batteries. Sodium-sulfur batteries are efficient and, relatively, cheap. They do require the sodium to be molten. The battery electrolyte must be electrically heated to greater than 300° C. The batteries have an implicit safety hazard due to the combination of high temperatures and the reactivity of molten sodium. This technology requires utility scale inverters to convert DC (direct current) to AC (alternating current). There is only one commercial producer of these batteries in the world. (Advanced Sodium-Sulfur (NAS) battery systems by Tokyo Electric Electric Power Company, Japan).
Compressed air storage (CAS) is the third technique for energy storage. This system requires two critical components: large scale gas compressors that can be driven by an electric motor and one or more subterranean caverns. These systems can be built in areas where the local geology has produced underground caverns. The cavern must be sufficiently large to storage thousands of cubic meters of compressed air. The systems are limited to locations with this resource. Compressed air could be stored in pressure vessels; however, the storage density is low, thus many large and expensive pressure vessels would be required. Furthermore, there is an inherent inefficiency in this storage mechanism. The act of compressing air results in the creation of heat. This is irreversible work and is a permanent loss of the system.
Energy also can be stored in a flywheel. In this case, input power is used to increase the rotational speed of the flywheel. The speed and inertia of the flywheel determines the amount of energy stored. These systems have a significant challenge in that the flywheels require exotic composite materials. The systems are capital intensive to scale up to the MW-hr range required for utility level power storage.
Alternate energy could be used to electrolytically split water to create oxygen and hydrogen. The hydrogen could be separated and stored at elevated pressure. Unfortunately, this technique is not suitable for use with underground storage. Hydrogen creation has all the same disadvantages of CAS with the added disadvantage that expensive, above-ground high pressure vessels are required to store the gas. Moreover, because hydrogen is such a small molecule, the above ground storage tanks and associated piping and valves must be carefully engineered to limit the leak down rate.
Supercapacitors are similar to battery storage in that energy is stored as an electrical charge within the system. Under the present state of the art, supercapacitors are suitable for short term energy storage over a period measured in seconds. At present, state of the art units from Maxwell Technologies are more expensive than batteries when evaluated for their storage potential (A-hr). The devices are suitable for short term energy storage where the charge and discharge cycle is on the order of seconds and minutes. Supercapacitors have an advantage over batteries as they have a nearly infinite life and can be charged and discharged without capacity degradation.
There is an existing technology for peak shaving that uses ice as a storage means. The motivation behind this technology is to use off-peak power to storage energy and minimize the consumption of power during times of peak consumption. The medium for energy storage is ice. Commercial systems are available for sale, such as that marketed by Trane and Ice Energy. During the off-peak period (typically at night), electricity is used to drive a refrigerant compressor. The cold output of the refrigerant compressor is used to freeze water to form ice. During peak power demand (i.e. the dead of the day), a heat transfer medium is passed through the ice. The ice cools the heat transfer medium. The heat transfer medium is used, in turn, to cool building air. Thus this system is focused on avoidance of peak power consumption for powering an air conditioner. This system does not generate electrical power. It allows power to be consumed at off-peak times to offset a power load associated with cooling at a peak time period. This would work for an office or industrial building. This form of energy storage does not enable the energy that is stored in the ice to be recovered as electric current.